Impact of helium ion energy modulation on tungsten surface morphology and nano-tendril growth

Woller, K. B.; Whyte, D. G.; Wright, G. M.

doi:10.1088/1741-4326/aa67ac

Title: Impact of helium ion energy modulation on tungsten surface morphology and nano-tendril growth

Journal Article · Wed Apr 12 00:00:00 EDT 2017 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/aa67ac· OSTI ID:1535585

Woller, K. B. ^[1]; Whyte, D. G. ^[1]; Wright, G. M. ^[1]

Massachusetts Institute of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center

Here, time-modulated helium (He) ion energy (e.g. V_Bias= –50 + 25·sin(2πf_RF · t), f_RF =13.56 MHz) is demonstrated to strongly affect the development of tungsten (W) surface morphology that results from He plasma irradiation in the DIONISOS linear plasma experiment. Nano-tendril bundles (NTBs), which appear as isolated 'islands' of nano-tendrils, can rapidly grow on an otherwise smooth W surface. This is in contrast to previously seen full-surface coverage of nano-tendril growth known as 'fuzz'. When tall NTBs form, less than 15% of the surface contains nano-tendrils. The NTB surface coverage changes with growth conditions and the total volume of nano-tendrils in the NTBs is observed to be up to a factor of 16 larger than when fuzz is grown. This indicates that long-range W surface transport underlies nano-tendril formation. Surface temperature 870–1220K, the DC bias potential –30 to –70 V, and the ion flux density 4.4 x 10²¹–1.1 x 10²² He · m^–2 · s^–1 are varied in the experiments. NTBs form at similar conditions as fuzz with the critical difference being the RF modulation of the ion energy bombarding the W, another indication of the importance of W surface transport. Mass loss measurements indicate net erosion with a yield of 1–8 x 10^–4 W/He when NTBs form; erosion that is not attributable to chemical or physical sputtering by He or impurities in the plasma. The erosion is correlated to the NTB growth, based on post-exposure inspection by electron microscopy indicating that NTBs are prone to loss from the surface. NTB growth is compared to the empirical growth-erosion model of fuzz, showing NTBs grow up to a factor of 100 times taller than the expected fuzz layer depth under DC bias conditions. Insights into nano-tendril growth provided by this new growth regime are discussed. Strategies to mitigate W fuzz growth may inadvertently result in rapid localized nano-tendril bundle growth with a higher probability of dust production.

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Research Organization:: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES); National Science Foundation (NSF)

Grant/Contract Number:: FC02-99ER54512; SC0002060

OSTI ID:: 1535585

Journal Information:: Nuclear Fusion, Vol. 57, Issue 6; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 39 works

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He-ion induced surface morphology change and nanofuzz growth on hot tungsten surfaces Meyer, F. W. Journal of Physics B: Atomic, Molecular and Optical Physics, Vol. 52, Issue 1 https://doi.org/10.1088/1361-6455/aaf060	journal	December 2018
Helium flux effects on bubble growth and surface morphology in plasma-facing tungsten from large-scale molecular dynamics simulations Hammond, Karl D.; Naeger, Ian V.; Widanagamaachchi, Wathsala Nuclear Fusion, Vol. 59, Issue 6 https://doi.org/10.1088/1741-4326/ab12f6	journal	May 2019
Helium plasma induced nanostructure formation in copper and nickel Thompson, M.; Song, K.; Kluth, P. Surface Topography: Metrology and Properties, Vol. 7, Issue 1 https://doi.org/10.1088/2051-672x/aaf209	journal	January 2019
Enhanced growth of large-scale nanostructures with metallic ion precipitation in helium plasmas Kajita, Shin; Kawaguchi, Shota; Ohno, Noriyasu Scientific Reports, Vol. 8, Issue 1 https://doi.org/10.1038/s41598-017-18476-7	journal	January 2018

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